1. Field of the Invention
The present invention relates to fracturing fluids of the type used in well bore operations and particularly to a method for producing a gradual reduction in the viscosity of a fracturing fluid through the use of slightly water soluble, organic peroxides incorporated in the viscous fluid.
2. Description of the Prior Art
During hydraulic fracturing, a sand laden fluid is injected into a well bore under high pressure. Once the natural reservoir pressures are exceeded, the fracturing fluid initiates a fracture in the formation which generally continues to grow during pumping. The treatment design generally requires the fluid to reach maximum viscosity as it enters the fracture which affects the fracture length and width. The fracturing fluid can be accompanied by a propping agent which results in placement of the propping agent within the fracture thus produced. The proppant remains in the produced fracture to prevent the complete closure of the fracture and to form a conductive channel extending from the well bore into the formation being treated once the fracturing fluid is recovered.
Fluid viscosity relates to the gel's ability to place proppant, influence fracture geometry and enhance fluid loss characteristics. The fracturing fluid's viscosity is normally obtained by using suitable polymers, such as polysaccharides. To further enhance the viscosity, a crosslinking agent is frequently added to the fracturing fluid to gel the polysaccharide.
The recovery of the fracturing fluid is accomplished by reducing the viscosity of the fluid to a low value such that it flows from the formation under the influence of formation fluids and pressure while retaining the proppant within the fracture. This viscosity reduction or conversion is referred to as "breaking" and can be accomplished by incorporating chemical agents, referred to as breakers, into the initial gel.
In addition to the importance of providing a breaking mechanism for the gelled fluid to facilitate recovery of the fluid, the timing of the break is also of great importance. Gels that break prematurely can cause suspended proppant material to settle out of the gel before being introduced a sufficient distance into the produced fracture. Premature breaking can also result in a premature reduction in the fluid viscosity resulting in a less than desirable fracture length in the fracture being created.
On the other hand, gelled fluids that break too slowly can cause slow recovery of the fracturing fluid from the produced fracture with attendant delay in resuming the production of formation fluids. Additional problems can result, such as the tendency of proppant to become dislodged from the fracture, resulting in at least partial closing and decreased efficiency of the fracturing operation.
For purposes of the present application, premature breaking will be understood to mean that the gel viscosity becomes diminished to an undesirable extent before all of the fluid is introduced into the formation to be fractured.
Optimally, the fracturing gel will begin to break when the pumping operations are concluded. For practical purposes, the gel should be completely broken within a specific period of time after completion of the fracturing period. At higher temperatures, for example, about 24 hours is sufficient. A completely broken gel naturally flushes from the formation by the flowing formation fluids or can be recovered by a swabbing operation. In the laboratory setting, a completely broken, non-crosslinked gel is one whose viscosity is either about 10 centipoises or less as measured on a Model 50 Fann viscometer R1/B1 at 300 rpm or less than 100 centipoises by Brookfield viscometer spindle #1 at 0.3 rpm.
By way of comparison, certain gels, such as those based upon guar polymers, undergo a natural break without the intervention of chemical additives. The break time can be excessively long, however. Accordingly, to decrease the break time of gels used in fracturing, chemical agents are incorporated into the gel and become a part of the gel itself. These chemical agents include, for example, oxidants such as persulfate salts.
However, obtaining delayed breaks using oxidants, such as persulfate salts, to degrade polysaccharide viscosifiers has proved difficult. The rate of degradation usually depends on the temperature and the persulfate concentration. At temperatures above about 140.degree. F., the rate of degradation is difficult to control, often causing a significant loss of viscosity before completing proppant placement. The fracturing fluid loses a sizeable fraction of the persulfate when water from the fluid permeates away from the fracture through the porous rock of the formation matrix. The larger polysaccharide simultaneously filters out and adsorbs onto the fracture face, forming a tough, leathery filter cake. Consequently, the relative concentration of polysaccharide steadily increases in the fracture during injection. This filtration effect leaves inadequate amounts of persulfate to degrade both the gel and filter cake. To combat the loss of persulfate to the formation, large amounts of persulfate are used. Large amounts of persulfates, however, can prematurely reduce the fluid viscosity during injection to cause a premature breaking of the gel.
In response to the problems with using oxidants, a recent advancement encapsulates persulfate to segregate it from the fracturing fluid. The persulfate either permeates through the capsule wall or is released by crushing from the proppant during fracture closure, thus delaying gel breaking. Both releases ensure that adequate persulfate is available for complete polysaccharide degradation.
The encapsulated persulfate has some disadvantages. The effectiveness diminishes with increasing temperature. At temperatures exceeding 200.degree. F., most of the persulfate is consumed within the capsule before being released. A pressure differential may form across the coating and force water into the capsule. At high temperature, water reacts with the persulfate within the capsule, thus decreasing the amount of persulfate available. Also, persulfate particles are angular and the capsule thickness can vary across the surface of the particle. This non-uniform thickness causes some thin skinned particles to release too early, causing premature viscosity loss.
The present invention has as its object to provide a break mechanism for a gelled fracturing fluid which yields high initial viscosity with little change during pumping but which produces a rapid break in the gel after pumping is completed to allow immediate recovery of the fluid from the formation.
Another object of the invention is to provide a gel system for a well fracturing operation which can break the gel polymers at moderate to high temperatures without interfering with the crosslinking chemistry and causing premature breaking.